3,3′-Dimethyl-1,1′-(butane-1,4-diyl)diimidazolium bis(tetrafluoroborate)

The title compound, C12H20N4 2+·2BF4 −, was prepared by the anion exchange of a dibromide ionic liquid with sodium tetrafluoroborate. The asymmetric unit contains one half of the imidazolium cation, which lies about an inversion centre at the mid-point of the central C—C bond of the linking butyl chain. The two planar imidazole rings (r.m.s. deviation = 0.0013 Å) are strictly parallel and separated by 2.625 (7) Å [vertical distance between the centroids of two imidazole rings], giving the molecule a stepped appearance. In the crystal structure, intermolecular C—H⋯F hydrogen bonds link the cations and anions, generating a three-dimensional network.


Comment
Ionic liquids (ILs) are generally formed by an organic cation and a weakly coordinating anion. They have enjoyed considerable research interest in recent years because of their unique properties such as high thermal stability, non-volatility, nonflammability, high ionic conductivity, a wide electrochemical window and miscibility with organic compounds (Welton, 1999;Nicholas et al., 2004;Yu et al., 2007). ILs have been widely applied to several areas including catalysis, electrochemistry, separation science, as solvents for green chemistry, biology and materials for optoelectronic applications (Olivier & Magna, 2002). Geminal dicationic ionic liquids have been shown to possess superior physical properties in terms of thermal stability and volatility compared to traditional ionic liquids (ILs) (Leclercq et al., 2007;Payagala et al., 2007) .
We here report the crystal structure of the title compound (I).
The atom-numbering scheme of (I) is shown in Fig.1, and all bond lengths are within normal ranges (Allen et al., 1987).
In the crystal structure intermolecular C-H···F hydrogen bonds link the cations and anions generating a three-dimensional network. (Table 1 and Fig.2). ).
A solution of above mentioned dibromide ionic liquid (3.8 g, 0.01 mol) in methanol(20 ml) was slowly added to a solution of sodium tetrafluoroborate (2.2 g, 0.02 mol) in methanol (20 ml), The reaction mixture was refluxed for 1 h. After evaporation of the solvent, the residue was washed with diethyl ether, then dried in vacuum to obtain title compound (I),

Refinement
All H atoms were positioned geometrically, with C-H = 0.93 Å, and constrained to ride on their parent atoms, with U iso (H) = xU eq (C), where x= 1.5 for methyl H and x = 1.2 for methylene H atoms.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.